The present invention relates generally to inkjet printing mechanisms, and more particularly to a bulldozer-type cleaner for removing ink residue from an electrostatic drop detector which detects ink droplets contacting the detector.
Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as xe2x80x9cink,xe2x80x9d onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a xe2x80x9cservice stationxe2x80x9d mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. To facilitate priming, some printers have priming caps that are connected to a pumping unit to draw a vacuum on the printhead. During operation, partial occlusions or clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a clearing or purging process known as xe2x80x9cspitting.xe2x80x9d The waste ink is collected at a spitting reservoir portion of the service station, known as a xe2x80x9cspittoon.xe2x80x9d After spitting, uncapping, or occasionally during printing, most service stations have a flexible wiper, or a more rigid spring-loaded wiper, that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment based inks have a higher solids content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper.
Unfortunately, occasionally a printhead nozzle becomes permanently damaged or blocked, so the nozzle is no longer able to eject ink. A missing nozzle cannot eject ink when directed to do so by the printer controller, leaving bare spots in the resulting image. Most earlier inkjet printers had no way of knowing when a nozzle was missing from the array, and the only way to improve print quality was to replace the defective printhead, often while the pen still contained a good supply of ink. Thus, there was a need to know when a particular nozzle was no longer functioning, and to fill this need a low cost ink drop detector was invented, as described in U.S. Pat. No. 6,086,190 to Schantz et al., currently assigned to the present assignee, the Hewlett-Packard Company. Use of the electrostatic drop detector provides a mechanism for communicating to the printer controller when a particular nozzle is out. Knowing this information, the printer controller may substitute a nozzle which is in good working order for the bad nozzle so print quality is unaffected by the missing nozzle. There are a variety of different ways this may be done, for instance using multi-pass print modes various shingling or mask routines, or other schemes known to those skilled in the art.
While several different types of electrostatic drop detectors are discussed in the Schantz et al. patent, several of the illustrated embodiments use an ink absorbing pad, such as a foam material, in conjunction with the electrostatic drop detector. The purpose of this foam is to absorb liquid components of the ink being spit onto the detector. However, as mentioned above, the current preferred electrostatic drop detector has a relatively smooth spit target surface, with no ability to absorb liquid components of the ink, or to dispel particulate matter of the ink composition. Indeed, droplets which are fired from functioning nozzles onto the drop detector may eventually build up over time, causing the detector to give inaccurate readings. In an extreme case, the ink residue may actually build up and form stalagmites. These ink stalagmites may eventually grow to a height where they could hit and damage the printhead, clogging nozzles or permanently destroying the printhead. Thus, it is apparent that an inkjet printing mechanism using such an electrostatic drop detection system needs some manner of addressing the ink residue build-up on the detector.